This application is the national stage application of PCT/EP98/00489, filed Jan. 30, 1998, which is entitled to priority of German Application No. 197 04 956.7, filed Feb. 10, 1997.
1. Field of the Invention
The invention relates to an arrangement and a method for measuring the direction of the geomagnetic field {right arrow over (B)}E in the proximity of a magnetic jamming device with a magnetic field of direction which can be varied as a function of time.
2. Description of the Related Art
The direction of the geomagnetic field serves in many applications as a reference for orientation and navigation on and above the Earth""s surface. The reference quantity is the direction of magnetic North, which is defined as the direction of the horizontal component of the geomagnetic field. A magnetic field measuring device DMC (Digital Magnetic Compass) developed by Leica AG, Heerbrugg, Switzerland, determines this component from the measurement of the magnetic field components in three dimensions in space and from the measurement of the horizontal position.
When the compass is installed in an electrical device, the measurement of the geomagnetic field is generally falsified by magnetic fields generated by magnetic parts of the device or current-carrying conductors. As long as these jamming devices are constant or variable only slowly as a function of time, they can be compensated by single calibration procedures or calibration procedures to be carried out over relatively large distances.
Thus, U.S. Pat. No. 4,539,760 discloses a method in which a vehicle equipped with a magnetic sensor system is brought in succession into different, predetermined orientations in space. A 3-D correction matrix for compensating magnetic interference is derived from position-dependent measured values. The method assumes interference which is constant as a function of time.
DE 38 33 798 A1 discloses a method for determining the interference effect of consumers which can be switched on and off on navigation systems. When consumers are switched off, a two-dimensional local curve of the geomagnetic field is first determined by turning the vehicle, the midpoint of which geomagnetic field is displaced by the vector of the magnetically hard interfering field present at the magnetic field sensor. Thereafter, with the vehicle stationary, the magnitude of the interfering effect of successively switched on consumers is determined and is used for correction when limits are exceeded. It is assumed that the interfering fields determined in the switched-on state always emit the same magnetically hard field. It is also necessary to know the instantaneous switching state of the jamming device.
If, however, the interference is rapidly variable, the known methods for compensating interfering fields cannot be used. A typical example of this is a device which contains a magnet which is mounted at a fixed point but is freely movable in all directions at this location, for example by means of a cardan suspension. Another example would be a coil through which a variable current flows.
An embodiment of the present invention is directed toward providing an arrangement and a method by means of which such a magnetic interference source variable as rapidly as desired can be eliminated with respect to its effect on the measurement of the direction of the geomagnetic field.
Advantageous embodiments are evident from the features described herein.